Photodetectors are commonly used in a number of solid state imaging devices, such as facsimile machines and solid state radiation detectors employed in medical diagnostic procedures, in which it is necessary to convert an optical image into corresponding electrical signals. For example, in a radiation imager the incident radiation strikes a scintillator material that generates light photons when the incident radiation is absorbed. Photodetectors in turn generate electrical signals when struck by the light photons, and the electrical signals are used to operate an imaging system or are further processed or analyzed.
Solid state photodiodes used in radiation imaging systems are typically arranged in either a one-dimensional or a two-dimensional array. The individual photodiodes are electrically coupled to a data electrode on or off of the substrate so that the charge developed in each diode in response to incident light can be sampled. In a typical photodiode a layer of photosensitive material is disposed between a common electrode and an electrical contact pad that is coupled to the data electrode. The change in charge in the photodiode resulting from exposure of the photosensitive material to the incident light photons creates an electrically measurable signal which is used in the imaging systems. Amorphous silicon is advantageously used as the photosensitive material.
The resolution of the imaging device is a function of the size and performance of each photodetector. To improve resolution, such as by presenting more image lines per inch of visual display, a greater number of photodiodes are required per unit area of the photodetector array. Particularly in a large array, such as one having an area about 16 square inches or larger, imager readout is degraded by an induced capacitance between the common electrode and the data lines to which the photodiode contact pads are coupled. Imager performance is also degraded if individual photodiodes have high charge leakage or "dark current", i.e. an electrical signal produced even when the photodiode is not exposed to light photons. It is important that the photodetector array be fabricated so as to allow photodiode charge to be accurately read within the allowed data sampling times of the system; it is thus desirable to minimize the capacitance between the common electrode and the data sampling lines and to provide a photodiode structure that reduces the undesired charge leakage. A critical step in the fabrication of the array that affects both of these concerns is the patterning of the photosensitive material into the individual structure associated with each photodetector, i.e., the photodiode body.
It is known to be difficult to form photodiodes from amorphous silicon, a desirable and commonly used photosensitive material, using traditional etching techniques. These difficulties manifest themselves both in problems related to consistent formation of individual photodiode bodies having a desired structure and in forming a large area array comprising substantially uniformly shaped photodiodes across the array. For example, it is difficult to reproducibly control the sidewall profile of the silicon photodiode body when using typical amorphous silicon etching techniques, such as use of a barrel etcher with an etchant comprising CF.sub.4 and O.sub.2. The sidewalls produced in silicon photodiode bodies etched by such methods frequently have a vertical or reentrant profile that can cause imager device defects due to poor step coverage of subsequently deposited layers. Additionally, because the barrel etcher provides a purely isotropic etch, there is poor dimensional control when etching a large number of photodiode bodies in an array due to the so-called "bulls-eye" effect in which structures at the edges of the array are etched more rapidly than structures towards the center of the array. Such a situation results in either the structures on the edges being overetched (i.e., made smaller than desired) or, if the etch time is reduced, the structures near the center of the array not being etched sufficiently.
Known reactive ion etching techniques also do not provide a reliable method for reproducibly and controllably etching silicon to create the structure desired in a photodiode body. The "resist erosion method" allows replication of the slope of a photoresist layer in underlying layers of certain materials to be etched, such as silicon dioxide, and this method is commonly used for fabricating via holes in semiconductor fabrication. See R. Saia and B. Gorowitz, "Dry Etching of Tapered Contact Holes Using Multilayer Resist", 132 J. Electrochem. Soc., 1954-57 (August 1985). Such methods, however, are not readily applied to etching amorphous silicon because the standard methods to match the resist and vertical silicon etch rates cause large increases in the lateral etch rate of the silicon, resulting in problems similar to those described above with respect to using the barrel plasma etcher. For example, if the vertical to lateral etch ratio of both the resist and the underlying layer being etched is high and equal for both materials, then the shape of the sidewall in the resist will be replicated in the etched layer. The steepness of the slope of the underlying material can be modified by changing the ratio of the vertical etch rates of both layers. In etching materials such as silicon dioxide, this change in etch ratios is typically accomplished by adding oxygen to a fluorinated gas (CHF.sub.3 /O.sub.2), or by altering the gas ratio to increase the amount of atomic fluorine in the plasma (NF.sub.3 /Ar), both of which raise the vertical etch rate of the photoresist with respect to silicon dioxide. These standard methods do not, however, produce acceptable results when etching amorphous silicon because the addition of the oxygen or alteration of the gas ratios also result in large increases in the lateral etch rate of the silicon, causing problems similar to those discussed above with respect to the use of barrel etching techniques.
It is thus desirable to have a method of etching amorphous silicon that allows the fabrication of an array of a large number of uniformly-shaped silicon photodiode bodies which have substantially uniformly sloped, smooth sidewalls allowing for improved step coverage of subsequently deposited layers and low charge leakage from the photodiode body. Additionally, such a silicon etch process desirably is selective to the underlying material layers which will be exposed to the etchant, such as silicon nitride or silicon oxide, so that such layers form an effective etch stop. The etch process also should be slightly isotropic in nature so as not to leave silicon spacers on the sidewalls of the nonplanarized underlying topography in the array; such residual silicon remnants or "stringers" could provide a path for electrical shorts which would adversely affect imager performance.
It is thus an object of this invention to provide a method of reproducibly and controllably patterning silicon to allow the formation of sidewall surfaces having a slope that is substantially uniform and the degree of which can be selected. A further object of this invention is to provide an effective and readily performed method of patterning amorphous silicon to form a large number of substantially uniformly shaped photodiode bodies in a photodetector array having the necessary characteristics to provide electrical signals for high resolution images of the detected light. It is yet a further object of this invention to provide a photodector structure having a photodiode body conducive to improved step coverage of subsequently deposited layers.